Printers for media generally include a printhead within a print zone where media is fed into alignment beneath the ink nozzles, jet or sprayers of the printhead. The media to be printed for example a postal code or stamp to be printed on a #10 envelope or envelopes of varying thicknesses, are provided in a loading area and are supplied through the print zone one at a time to a printhead where print indicia such as a cancellation stamp can be applied by the printhead to the appropriate portion of the media. The printhead can be any kind of ink or laser jetting printing assembly. The printhead generally includes a plurality of ink emitting nozzles from which a selected color ink is jetted or adhered to the media. Print quality is improved if the print media is allowed to be as close as possible to the printhead. The clearance distance between the front side control surface of the print media and the print nozzles is critical because all ink jetting printheads exhibit a certain amount of spray as small stray drops of ink are ejected slightly off trajectory from the main drop. If the distance between the front side control surface of the print media and the ink jetting nozzles is small, stray ink drops are minimized.
However, the clearance distance between the ink jetting nozzles and the front side of the print media can only be minimized to a certain extent. If the clearance distance is too small, waving in the print media caused by wet ink may cause the front side of the print media to touch the printhead in the area of ink jetting nozzles. When the print media is touching the printhead, the ink drops of course cannot be properly jetted on the front side thereof for formation of a print image. If the media is too far away from the printhead there will of course be blurring and overspray from the printhead nozzles.
Conventional systems which control the clearance distance between the print media and the printhead are of two basic types. The first type uses a back side control surface which is disposed at a predetermined distance away from the printhead. The print media is forced against the back side control surface. The distance between the front side control surface and the printhead thus varies depending on the thickness of the print media. Since the back side control surface is fixed, the clearance distance between the front side of the print media in the printhead must be sufficiently large such that the print media will not contact the printhead as described above. Decrease in clearance distance to improve print quality may result in the print media contacting the printhead which is not desirable as described above.
The second general type of system used to control the clearance between the print media and the printhead biases the print media against the front side control surface as opposed to the backside control surface. The front side control service may either be movable in transverse directions along with the printhead, or be fixed and extend across the width of the print media. In either event, the clearance distance between the edge of the front side control surface and the print media is fixed and does not change regardless of the type of print media used during printing.
For example, U.S. Pat. No. 5,648,807 (Saito et al.) discloses an inkjet printer (FIG. 3A) having a paper feed roller 330 which is engaged by a pinch roller 350. Pinch roller 350 is rotatably attached to the distal end of the paper guide 53 which is suspended from a rear, fixed frame 130 using a spring 52 so that paper guide 53 rotates about a fulcrum point 51. Frame 130 not only interconnects with the paper guide 53 but also substantially forms an enclosure which carries the plurality of gears, rollers etc. (FIG. 18). As shown in FIG. 27 a lower end of the rear frame 130 is attached and carries a pressing member 140 which is disposed above feed roller 330. Because of the fixed nature of frame 130, pressing member 140 is always “located at a slightly lower position from a tangent T to both feed roller 330 and transport roller 381, and is arranged to press paper P downward.” Because of the fixed and immovable nature of frame 130 and pressing member 140, pressing member 140 does not move with or relative to pinch roller 350 carried by paper guide 53, but rather is fixed in a stationary position.
As a further example, U.S. Pat. No. 6,089,773 discloses a media feed system for an inkjet printer used to control the distance of the media from the printhead. Four deflector plate assemblies 22 (FIGS. 1-4) are successively arranged across the width of the print medium. Each deflector plate assembly 22 includes an elongate base 34, at least one metering roller 36 and at least one deflector plate 38. An extension 40 of the elongate base 34 is attached to a tension spring 42 that biases the elongate base 34, metering rollers 36 and deflector plate 38 towards the feed roller 20. Each metering roller is positioned in association with and defines a nip 50 with feed roller 20 through which print medium 12 passes. “Print medium 12 is engaged by feed roller 20 and is carried though nip 50 formed with metering rollers 36. Metering rollers 36 and deflector end 46 of deflector plate 38 are moved away from feed roller 20 a gap distance (not numbered) which is associated with the thickness of print medium 12. Depending upon the force applied by tension spring 42, metering rollers 36 may slightly compress print medium 12 such that the gap distance is slightly less than the thickness of print medium 12. Likewise, the compressive force applied to print medium 12 in nip 50 by metering rollers 36 may result in a slight cupping of feed roller 20, depending upon the material from which feed roller 20 is constructed.” Either result may be poorer quality print reproductions or damage to the print media. What is particularly needed in the art is a media feed system which overcomes the problems associated with fixed front side and back side media control surfaces and which can account for variable thickness media within the same printing run.